[PATCH] kernfs: avoid READ_ONCE() on a kgid_t in kernfs_new_node()

[PATCH] kernfs: avoid READ_ONCE() on a kgid_t in kernfs_new_node()

[Shakeel Butt]

kernfs_new_node() reads the parent's gid with READ_ONCE(attrs->ia_gid).
That gid is a kgid_t, which is a struct.

On SMP alpha, READ_ONCE() ends by casting the value it loaded back to
the type of its argument. Here that is a cast from a struct to the same
struct. GCC accepts that as an extension, but ISO C does not allow a
cast to or from a struct type, so sparse warns:

  fs/kernfs/dir.c:751:31: sparse: sparse: cast to non-scalar
  fs/kernfs/dir.c:751:31: sparse: sparse: cast from non-scalar

Read the gid_t inside the kgid_t instead, and wrap it back up with
KGIDT_INIT(). This is still a single marked load of the same four bytes.

There is no functional change. With GCC 12 for alpha (the config from
the report) and GCC 14 for x86-64 (with and without KCSAN),
fs/kernfs/dir.o has the same object code before and after this change.

Fixes: 81bdc85b4c08 ("kernfs: fix up the unlocked attribute reads on the creation paths")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202610261608.vItiXCGW-lkp@intel.com/
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 fs/kernfs/dir.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 324d61a00545..800af10d9511 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -750,7 +750,7 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
 		struct kernfs_iattrs *attrs = READ_ONCE(parent->iattr);
 
 		if (attrs)
-			gid = READ_ONCE(attrs->ia_gid);
+			gid = KGIDT_INIT(READ_ONCE(attrs->ia_gid.val));
 
 		if (flags & KERNFS_DIR)
 			mode |= S_ISGID;
-- 
2.53.0-Meta

Re: [PATCH] kernfs: avoid READ_ONCE() on a kgid_t in kernfs_new_node()

[Tejun Heo]

On Tue, Oct 27, 2026 at 07:32:47AM -0700, Shakeel Butt wrote:
> kernfs_new_node() reads the parent's gid with READ_ONCE(attrs->ia_gid).
> That gid is a kgid_t, which is a struct.
> 
> On SMP alpha, READ_ONCE() ends by casting the value it loaded back to
> the type of its argument. Here that is a cast from a struct to the same
> struct. GCC accepts that as an extension, but ISO C does not allow a
> cast to or from a struct type, so sparse warns:
> 
>   fs/kernfs/dir.c:751:31: sparse: sparse: cast to non-scalar
>   fs/kernfs/dir.c:751:31: sparse: sparse: cast from non-scalar
> 
> Read the gid_t inside the kgid_t instead, and wrap it back up with
> KGIDT_INIT(). This is still a single marked load of the same four bytes.
> 
> There is no functional change. With GCC 12 for alpha (the config from
> the report) and GCC 14 for x86-64 (with and without KCSAN),
> fs/kernfs/dir.o has the same object code before and after this change.
> 
> Fixes: 81bdc85b4c08 ("kernfs: fix up the unlocked attribute reads on the creation paths")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202610261608.vItiXCGW-lkp@intel.com/
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun